Few Americans have spent more one-on-one time with the leading Republican presidential candidates than Donald Trump.

The real estate mogul and reality TV host has devoted the second half of this year to fashioning himself as the single most important potential endorser in the GOP sweepstakes. The candidates have cycled through his offices at Trump Tower to ask for his support, and he has cycled through Fox News’s anchors talking about it. The latest to come calling is former House Speaker Newt Gingrich, R-Ga., who will visit on Monday.

This is what makes the afterword of his new book, “Time to Get Tough,” so relevant. After boasting of his own accolades — he writes, “I was quickly #1 in the polls,” as well as, “I am a ratings machine” — Trump offers his personal impressions of the candidates.

Trump writes that Mitt Romney has been “spectacular” in the debates and that the former Massachusetts governor is “a much different guy” in private than he is in public, describing him as “warm and engaging.”

“He gets criticized for changing his opinions, or ‘flip flopping,’ but over a lifetime I’ve seen many people who don’t change and they always get left behind,” Trump writes. “Smart people learn things, so they change their minds. Only stupid people never change their minds.”

In an interview, Trump repeated his defense of Romney’s evolving positions on key issues. “If you have a concrete wall in front of you, you can’t just try to run through it,” Trump said. “You have to go around it, under it, over it. Everybody changes.” Trump writes that he found Texas Gov. Rick Perry to be “much different from what you see in the debates . . . so forceful and strong.”

Trump writes that he said to Perry, “Rick, why can’t you act this way in the debates?”

“He said, ‘Donald, the debates are just not my thing. So I said, ‘Why don’t you pretend you are someplace else? You gotta act different. You are getting killed in the debates.’ But he repeated, ‘Donald, they are just not for me.’ ”

Trump describes Rep. Michele Bachmann, R-Minn., as “a real worker bee.” He notes that she visited his office “more than once,” and that “no matter what happens with her run for the White House, she’s got a great political future ahead of her.”

One candidate who hasn’t visited Trump is former Utah Gov. Jon Huntsman Jr.

Huntsman has made a public show of not wanting or needing to see Trump, and last week he declined an invitation to a debate in Iowa that Trump is moderating two days after Christmas.

But in his book, Trump tells a different story. He writes that Huntsman had courted Trump just like the other candidates.

“He called me a number of times and I was unbelievably busy doing a deal and I didn’t get back to him,” Trump writes, adding: “In fact, he left me his number and the person’s name to call to set up a meeting.”

Trump writes nothing about Rep. Ron Paul, R-Texas, who spurned him Sunday on CNN: “I don’t understand the marching to his office. I mean I didn’t know that he had an ability to lay on hands, you know, and anoint people.”

In the interview, Trump said he plans to endorse a candidate “over the next month or so” and that he would consider making an independent run next year if he is not satisfied with the Republican nominee.

Trump weighed his own run this spring, as he drew media attention for challenging President Barack Obama’s birthplace. He writes that he didn’t run because he believed the press would have unfairly gone after his hundreds of business deals. “It is very hard for a truly successful person to run for office,” he writes.

But what really gave Trump a rise was Obama’s riff on him at this year’s White House Correspondents’ Dinner, which Trump attended as a guest of The Washington Post. The president was making fun of Trump and the outsize role he was playing in the GOP race.

“While I shouldn’t admit this, I don’t mind being the center of attention, especially on such an evening,” Trump writes, adding: “I loved it!”